1. Field of the Invention
The present invention relates to a spiral wound gas permeable membrane module using a hydrophobic gas permeable membrane and an apparatus and a method for using the same.
2. Description of the Prior Art
Generally, gases such as oxygen, nitrogen and carbon dioxide are dissolved in a water, and sometimes these dissolved gases give a bad influence to a water treatment. For example, oxygen dissolved in a water often accelerates the corrosion of the inner surface of a pipe wetted by the water in a water circulation system, and carbon dioxide dissolved in a water often deteriorates the water quality of a super pure water produced by a water purifying system. In such cases, degasification treatment has been performed as required by, for example, addition of chemical or vacuum treatment as disclosed in, for instance, JP-A-SHO 58-14905, SHO 58-101784 and SHO 58-186490.
In the conventional degasification by such a chemical treatment, however, there are problems such as the cost of chemical and residual components Also there are restrictions such as the cost of the system and the running cost even in the vacuum treatment. Therefore, these conventional treatments have not truly served for a desired practical degasification.
On the other hand, recently a method for degasifying dissolved gases from a raw water by using a hydrophobic membrane having a gas permeation property has been developed for practical use (for example, Japanese Utility Model Laid-Open SHO 57-35795, JP-A-SHO 62-273095). In this method, a raw water flows on the surface or back surface of a membrane having a gas permeation property and a hydrophobic property, the other surface side being controlled to the condition of a reduced pressure, and thereby only the gas dissolved in the raw water is permeated through the membrane and removed from the raw water. This method has the advantages that there is no residual chemical left which is the problem in the conventional chemical addition method and that the apparatus or system for this method is simple and the running cost thereof is inexpensive as compared with the conventional vacuum degasification method.
In degasification using a hydrophobic gas permeable membrane, the membrane forms a unit of a module. As the types of such gas permeable membrane modules, the spiral type and the hollow yarn type are well known, depending upon the formation of the membrane (for example, JP-A-HEI 2-2802, Japanese Utility Model Laid-Open HEI 2-25096). In a case where the amount of gas to be degasified is large, it is considered that a spiral type module using a plane gas permeable membrane is good. A typical conventional spiral module is constructed, for example, as shown in FIG. 10. The spiral module shown in FIG. 10 basically has the same structure as those disclosed in JP-B-SHO 44-14216, JP-A-SHO 54-31087 and JP-A-SHO 56-129006 which are used for the separation of liquids In FIG. 10, the unit comprises an envelope-like hydrophobic gas permeable membrane 105 sealed at both of its side edges 102, a permeation side spacer 106 provided in the envelope-like membrane and a feed water spacer 104 on the outer surface of the envelope-like membrane, and one or a plurality of the units are wrapped around a hollow mandrel 101 which has a plurality of holes on its surface and whose one end is plugged and the other end 101a is opened. Raw water 107 is supplied to the outer surface side of envelope-like membrane 105, and the gas pressure in the envelope-like membrane is reduced by the suction by a pressure reducing source connected to opening end 101a of hollow mandrel 101 to cause a pressure difference between the surface and the back surface of hydrophobic gas permeable membrane 105. The gas dissolved in raw water 107 permeates from the surface of envelope-like membrane 105 to its back surface, and the permeated gas 103 flows along the passageway of permeation side spacer 106 in the envelope-like membrane towards the hollow mandrel 101. Thus, the dissolved gas is removed from the raw water 107.
In such a degasification, however, since the degasification ability greatly depends on the difference between the partial gas pressures of the surface side and back surface side of the membrane as well as on the specific gas permeation property of the membrane itself, it is necessary to reduce the partial gas pressure of the gas permeation side by elevating the degree of vacuum on the gas permeation side of the membrane, or to increase the supply pressure of the raw water supplied to the module, i.e., to the surface side of the membrane, in order to increase the degasification ability of the module. However, the degree of vacuum of the gas permeation side and the supply pressure of the raw water are both limited to certain levels from the viewpoint of practical use, therefore it is difficult to further increase the degasification ability.